Ceramic materials are well-known and widely used for different applications due to some of their exceptional properties, such as relatively high modulus, hardness, high temperature stability, and/or chemical resistance. Ceramic materials can also be, however, relatively heavy, brittle, and/or difficult to process. Alternatively, organic polymers can be relatively tough, flexible, light, and/or easy to fabricate and process, but their relatively low moduli and relatively low decomposition temperatures prevent their use in some applications. Pre-ceramic polymer technology is emerging as a promising approach for producing materials that share the advantages of both polymers and ceramics, while minimizing the disadvantages.
Mixed organic/inorganic polymer compositions have been prepared (for example, by the hydrolysis of tetraalkoxysilanes containing polymerizable organic groups) to circumvent the insolubility of many important engineering polymers within sol-gel solutions. Curing of such sol-gel processed monomers has provided mixed systems exhibiting some of the properties of the organic components, as well as some of the properties of the inorganic components. Such mixed systems have typically comprised semi-interpenetrating networks composed of linear organic polymers and a three-dimensional silicon dioxide network.
Many polymers are known to act as ceramic precursors, and their use for production of ceramic structures has been reported. Polysilazanes and modified polysilazanes (for example, isocyanate-modified, isothiocyanate-modified, thiourea-modified, boron-modified, peroxide-modified, and amide-modified) have been prepared and used for pyrolytic conversion to a ceramic material (for example, silicon nitride). Polysilazanes have also been used to modify materials such as epoxy resins, phenolic resins, and polyamines.
Hybrid organic/inorganic polymers or ceramers (including hybrid polysilazane polymers or ceramers) have been prepared by the reaction of organic electrophiles with metal-containing polymers. The hybrid polymers are said to comprise organic segments derived from the organic electrophiles and inorganic fractions derived from segments of the metal-containing polymers. Such hybrid polymers have been proposed for use as coatings on substrate materials, for molding applications (with or without fillers), and for other polymer applications in which their hybrid properties (for example, a combination of relatively high mechanical strength and high temperature stability) can be advantageous.
Curable perhydropolysilazane (inorganic homopolymer) and curable polyorganosilazanes (homopolymers or copolymers composed of organo-modified silazane units) have been prepared by ammonolysis of various dihalosilanes and organodihalosilanes, respectively. In addition, one example of a curable copolysilazane (hybrid organic/inorganic copolymer, with only a portion of its silazane units being organo-modified silazane units) has been prepared by ammonolysis of a combination of dichlorosilane and methyl dichlorosilane.
Such curable polysilazanes can have main chains or backbones that comprise structural units having the following general formula:—[Si(Ra)(Rb)—N(Rc)]—  Formula Iwherein each Ra, each Rb, and each Rc is independently hydrogen, an organic group, a heteroorganic group, or a combination thereof. In perhydropolysilazane, all of Ra, Rb, and Rc in Formula I above are hydrogen, and, in the other polysilazanes, at least one of Ra and Rb is a group other than hydrogen for a portion (copolysilazane) or all (polyorganosilazanes) of the structural units.
In spite of the numerous drawbacks of pyridine use (for example, relatively high odor, cost, hydrophilicity, and boiling point), the methods of preparing the perhydropolysilazane and the copolysilazane typically have involved the use of relatively large excess amounts of pyridine (for example, at least about 12 times the stoichiometric amount of silicon-halogen bonds in the starting silanes). These large excesses have been used to form pyridine adducts of apparently all starting silanes in situ, and the resulting pyridine adducts have then been co-ammonolyzed in the presence of the remainder of the large excess of base.